I had a busted valve spring on a head recently purchased EB head. They replaced the head and after the install I've noticed a drop in MPG. Any ideas? I watch my MPG religiously with the attempts of improvement but have been stuck at 20 MPG but have dropped to 18.5 The motor sounds and runs good. Timing is proper. I haven't checked compression but have no reason to expect any red flags with that. I've done plenty of head/head gasket replacements with positive outcomes. But this is the first time I've noticed a drop in MPG. I haven't checked the valves since the adjustment during the install but I would think they would have to be way off to effect MPG.

 

A few things may be the culprit.

If the valve timing and lash isn't exactly the same you could see a loss in mpg.

Also if your previous head was decked at any point, the new (tall) head would have lower compression.

Ignition timing difference from before vs after can effect mpg.

Power and performance loss from a tight new valve train.

Damaged injector during reassembly.

There are a lot of things that could cause the drop

 

You didn't mention a camshaft replacement so I won't go there.


Are the valve sizes the same on the new head?


Perhaps the intake & exhaust runners on the new head are a little larger?
Using a thicker head gasket can also reduce compression and hurt efficiency.


Other than that what was already mentioned is a good guess.
Valve adjustment, timing retarded from where it was before head replacement, larger combustion chambers in the new head.

 

probably timing. the computer will adjust it, and might equate to a weak spark pattern offset in the cap, which in turn adjusts to different fuel map profile via 02 sensor adjustment.

 

I noticed the same thing when I replaced my 22RE head with a new one.
The old one had been decked and I think the slight difference in MPG was due to difference in Compression Ratio.

 

If it is not of a mechanical nature.

Perhaps your just driving a little harder putting your foot into it more and don`t notice.